The present invention relates to a 12CaO.7Al2O3 compound which is an oxide clathrating an O2xe2x88x92 ion radical and an Oxe2x88x92 ion radical as active oxygen species in a high concentration (hereinafter, these two ion radicals are referred collectively to as xe2x80x9cactive oxygen speciesxe2x80x9d). The present invention also relates to a method for producing such a compound and to the use thereof.
An O2xe2x88x92 ion radical is known as one of active oxygen species which has a key role in oxidizing processes of various organic and inorganic materials. Extensive researches have heretofore been made on O2xe2x88x92 absorbed on the solid surface of an oxide compound (J. H. Lunsford, Catal, Rev. 8, 135, 1973, M. Che and A. J. Tench, Adv. Catal, 32, 1, 1983). In most of such researches, high-energy gamma rays are irradiated onto the surface of an oxide compound to create O2xe2x88x92 ion radicals thereon.
RO2 (R:alkali metal) is known as a crystal including an O2xe2x88x92 ion radical as a constituent anion. However, the related compounds are unavailable for a certain application such as oxidation catalysts or ionic conductors, because all of the compounds will be readily decomposed even at a temperature of 300xc2x0 C. or less.
As compared to O2xe2x88x92 ion radicals, Oxe2x88x92 ion radicals have higher activity. Several articles have reported that a small amount of Oxe2x88x92 ion radicals was included in alkali halide glasses, calcium-aluminosilicate glasses or the like (J. R. Bralsford et al., J. Chem. Physics, Vol. 49, pp 2237, 1968, H. Hosono et al., J. Am. Ceramic. Soc., 70, 867, 1987). However, there has not been known any crystal having an Oxe2x88x92 ion radical as a constituent ion.
In 1970, H. B. Bartl et al. made a point that among sixty-six oxygens within a unit cell containing two molecules in a 12CaO.7Al2O3 crystal, so-called C12A7, two oxygens of them existed within a space of each cage structure in the crystal as xe2x80x9cfree oxygensxe2x80x9d without residing in a network of the crystal (H. B. Bartl and T. Scheller, Neuses Jarhrb. Mineral., Monatsh, 1970, 547).
Based on an electron spin resonance analysis, Hosono, one of the inventors, et al. have discovered that about 1xc3x971019 cmxe2x88x923 of O2xe2x88x92 was clathrated in a 12CaO.7Al2O3 crystal synthesized by reacting two raw materials, CaCO3 and Al2O3 or CaCO3 and Al (OH)2, in a solid phase reaction at a temperature of 1200xc2x0 C. in ambient atmosphere. They have proposed a model in which a part of free oxygens exists in each cage structure in the form of O2xe2x88x92 (H. Hosono and Y. Abe, Inorg. Che. 26, 1193, 1987).
12CaO.7Al2O3 is inherently a stable oxide having a melting point of 1415xc2x0 C. If a larger amount of active oxygen species can be clathrated in this oxide and then a reversible incorporation and release of oxygen can be achieved, the oxide would have a desirable availability for various purposes, such as oxidation catalysts or ionic conductors.
While one of the inventers, et al. have found out that O2xe2x88x92 was clathrated in the 12CaO.7Al2O3 crystal, the concentration of O2xe2x88x92 was a relatively low value of 1019 cmxe2x88x923 and any Oxe2x88x92 ion radical having higher activity has not been identified. Further, any effective technique for controlling the amount of O2xe2x88x92 and releasing/incorporating it from/into the crystal reversibly has not been achieved.
For using such a compound as high-efficiency oxidation catalysts or antibacterial agents, it is required to clathrate the active oxygen species in a higher concentration and to provide a reversible function for releasing the clathrated active oxygen species and incorporating oxygen from outside. It is also necessary to establish a technique for quantitatively analyzing the concentration of the clathrated active oxygen species.
The inventers have discovered that a 12CaO.7Al2O3 compound clathrating active oxygen species in a high concentration of 1020 cmxe2x88x923 or more is obtained by preparing a raw material including calcium and aluminum mixed with each other in an atomic equivalent ratio of approximately 12: 14 and then reacting the raw material in a solid phase reaction at a controlled temperature under a controlled atmosphere. The present invention is directed to such a compound itself, a method for producing the same, a method for releasing clathrated ions, and the use of the compound.
More specifically, the present invention provides a 12CaO.7Al2O3 compound produced by preparing a raw material including calcium and aluminum mixed with each other in an atomic equivalent ratio of approximately 12:14, preferably a raw material including calcium carbonate and gamma-aluminum oxide mixed with each other in a molecular equivalent ratio of approximately 12:7, and then reacting the raw material in a solid phase reaction at a sintering temperature of 1200xc2x0 C. or more, preferably 1300xc2x0 C., under an atmosphere with an oxygen partial pressure of 104 Pa or more and a water-vapor partial pressure of 102 Pa or less, preferably an oxygen partial pressure of 105 Pa or more and a water-vapor partial pressure of 1 Pa or less. This compound can include 1020 cmxe2x88x923 or more of clathrated active oxygen species. The amount of the clathrated active oxygen species can be determined by an electron spin resonance analysis and a Raman spectrum analysis.
When the sintering atmosphere is arranged in an oxygen partial pressure less than of 104 Pa and a water-vapor partial pressure of more than 102 Pa, the concentration of the clathrated active oxygen species will be less than 1020 cmxe2x88x923. Further, even under a dry oxidation atmosphere with an oxygen partial pressure of 104 Pa or more and a water-vapor partial pressure of 102 Pa or less, when the sintering temperature is arranged in less than 1200xc2x0 C., it will be difficult to synthesize the desired 12CaO.7Al2O3 compound. Conversely, when the sintering temperature exceeds 1415xc2x0 C., the raw material will be undesirably molten. Thus, it will also be hard to obtain the desired 12CaO.7Al2O3 compound. In case of synthesizing the 12CaO.7Al2O3 compound through a solid phase reaction, the mixture of calcium carbonate and gamma-aluminum oxide is suitable for the raw material. However, any combination of calcium hydroxide or calcium oxide and aluminum hydroxide or one of various aluminum oxides (alpha, gamma or theta aluminum oxide) may be used as the raw material to synthesize the above compound.
An electron spin resonance (ESR) spectrum (at 77 K) of the 12CaO.7Al2O3 compound clathrating the active oxygen species is formed of a superposition of two spectrums; one defined by gx=2.00, gy=2.01 and gz=2.04, the other defined by gx=gy=2.05 and gz=2.00. These g values correspond to those of O2xe2x88x92 ion radicals and Oxe2x88x92 ion radicals in the solid, respectively. Thus, it can be concluded that O2xe2x88x92 ion radicals and Oxe2x88x92 ion radicals are clathrated in the 12CaO.7Al2O3 compound.
The absorption band shape in the ESR spectrum is symmetric at room temperature and becomes asymmetric at a low temperature of 77K. This indicates that O2xe2x88x92 ion radicals and Oxe2x88x92 ion radicals rotationally move within each cage structures at room temperature, while they are coupled electrostatically with and retained spatially by Ca2+ ions residing on each wall of the cage structures at low temperature. Each concentration of O2xe2x88x92 ion radicals and Oxe2x88x92 ion radicals can be quantitatively determined from the intensity of the absorption band.
In a Raman scattering spectrum of the above compound, a strong scattering peak is exhibited around 1130 cmxe2x88x921. This peak corresponds to the peak of O2xe2x88x92 ion radicals which has been reported by K. Nakamoto et al. (K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compound, 1978, Wiley). Since there is a certain dependence between the ESR absorption band and the Raman scattering intensity, the intensity of clathrated O2xe2x88x92 ion radicals can be quantitatively determined from the Raman scattering intensity. When the above compound is heated at 1200xc2x0 C. or more under an oxygen partial pressure of 104 Pa or less or a water-vapor partial pressure of 102 Pa or more, the active oxygen species or oxygen molecules will be released from the compound.